clang warns about uninitialized variables @__cur_setting_name and
@__cur_property_name as used inside @__code. Before calling to @__code,
explicitly initialize the values.

    Making all in tests
    make[4]: Entering directory `./NetworkManager/libnm-core/tests'
      CC       test-general.o
    test-general.c:2730:24: error: variable '__cur_property_name' is uninitialized when used here [-Werror,-Wuninitialized]
                                  NMTST_VARIANT_DROP_PROPERTY (NM_SETTING_CONNECTION_SETTING_NAME,
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ../../include/nm-test-utils.h:1079:20: note: expanded from macro 'NMTST_VARIANT_DROP_PROPERTY'
                                && !strcmp (__cur_property_name, __property_name)) \
                                            ^~~~~~~~~~~~~~~~~~~
    ../../include/nm-test-utils.h:1023:3: note: expanded from macro 'NMTST_...